You need to concentrate really much in Quantitative Aptitude if you want to get 30+ marks in it.
Quantitative Aptitude is the most time consuming subject, in practice and also in the exam so you should make the speed of doing your calculations fast and try to solve questions as fast as you can. Do more and more practice of the questions as it is not sure which question comes in the exam. Most questions are of same format but the way of doing all of them is quite different. You need a good hold on the formulas so that you can increase your speed of performing the questions.
Speed, accuracy, practice and remembering power are the most important factors which help you succeed in quantitative aptitude and if you get a good control on these factors then you may be even able to score 50 out of 50 in quantitative aptitude.